1. Field of the Invention
The present invention relates to a liquid crystal light valve containing a photoconductor and light blocking layer. More specifically, the present invention relates to a light valve having a light blocking layer comprised of an amorphous germanium-containing or tin-containing alloy. Specifically, the light blocking layer may contain a mixture of an element from the group of germanium and tin, at least one element from the group of hydrogen, oxygen and nitrogen, and zero or more elements from the group of carbon and silicon. In the preferred embodiment, this light blocking layer contains an amorphous hydrogenated alloy of germanium and silicon.
2Summary of the Prior Art
The prior art is replete with liquid crystal light valves. These light valves are used in high resolution displays, electronic imaging and optical computing applications. With respect to the present invention, those light valves of most interest employ a photoconductor layer and operate in reflection mode. One light valve of this type is found in U.S. Pat. No. 4,019,807 for a Reflective Liquid Crystal Light Valve with Hybrid Field Effect Mode, issued Apr. 16, 1977, by Boswell et al. The device of this patent utilizes a cadmium sulfide CdS photoconductor, a cadmium telluride CdTe light blocking layer, a CdS/CdTe photoresponsive heterojunction, and a magnesium fluoride/zinc sulfide MgF/ZnS multilayer dielectric mirror.
As light valve technology has progressed, it has become apparent to those skilled in the art that hydrogenated amorphous silicon (hereinafter "a-Si:H") has significant advantages over CdS as a photosensitive layer, particularly with regard to speed of light valve operation and reproducibility. There exist in the prior art numerous publications describing light valves which utilize an a-Si:H photosensitive layer, but which have no light-blocking layer or dielectric mirror. Thus, light valves incorporating an a-Si:H photosensitive layer, a light-blocking layer, and a dielectric mirror are less common in the prior art.
U.S. Pat. No. 4,799,773 for a Liquid Crystal Light Valve, issued Jan. 24, 1989, by Sterling, describes an a-Si:H photoconductor light valve which uses CdTe as the light-blocking layer and a silicon dioxide/titanium dioxide SiO.sub.2 /TiO.sub.2 multilayer dielectric mirror. In this light valve, a special multilayer intermediate bonding structure is required to bond the CdTe light blocking layer to the CdS photoconductor layer. In the absence of this extraneous layer, peeling of the light blocking layer from the photoconductor layer, and vice versa, occurred. The extraneous multilayer structure also facilitated device repeatability. A significant disadvantage of this type of light valve structure, however, is that a rather complex and lengthy fabrication is required to produce the multiple and chemically unique layers.
More specifically, in the device of the Sterling patent, the special multilayer structure is required to bond the CdTe layer to the photoconductor because CdTe does not adhere well when directly deposited on the a-Si:H photoconductor. Fabrication of the bonding structure requires four processing steps and a dedicated thin film deposition system. In addition, separate thin film deposition systems are required for photoconductor layer deposition and CdTe deposition. Moreover, deposition of the CdTe light blocking layer must be carefully controlled to maintain precise CdTe stoichiometry so that the layer has a resistivity high enough for high resolution light valve applications.
The prior art light valve has performance disadvantages. It is desirable to construct a light valve having as thin a light blocking layer as possible. As will be explained, the thinner the light blocking layer is while satisfying the optical density requirement, as well as other requirements, the better the performance of the light valve system. According to the prior art preferred embodiment of Sterling, a CdTe light blocking layer of 2 micrometers thickness is required. The present invention is a light blocking layer which is easier to deposit than CdTe and which performs the same functions with a thinner layer.
The placement of amorphous alloys of silicon and germanium, and other elements, in contact with amorphous silicon is generally known, although not for use to form a light blocking layer in a photoaddressed liquid crystal light valve. Amorphous silicon germanium alloy has been deposited on amorphous silicon in tandem solar cells as a photovoltaic layer and in electrophotographic devices as a photosensitive layer, for example. The prior art of photocells and electrophotographic devices does not address or touch on a light blocker's three essential requirements of low impedance, high optical density, and high sheet resistivity because there is no need to maintain resolution of an optical signal or block light. Furthermore, the material characteristics necessary for amorphous silicon germanium alloys which are used in light blockers are different from the material characteristics necessary in photocells and electrophotographic devices.
U.S. Pat. No. 4,723,838 for a Liquid Crystal Display Device, issued Feb. 9, 1988, by Aoki et al, describes an amorphous silicon germanium alloy layer placed adjacent to a photosensitive silicon layer for the purpose of blocking light. There are substantial reasons why Aoki et al is not applicable to the technology of spatial light modulators and/or lacks a teaching necessary for the construction of a light blocking layer usable in a spatial light modulator.
The fact that amorphous materials are usable as light blockers in TFT matrices, as in Aoki et al, does not imply that they are usable as light blockers in spatial light modulators. In the light valve, the light blocking layer must simultaneously meet the following three critical factors to satisfy operating capability: the optical density must be high (3 OD or more) to achieve good light absorption the sheet resistance must be high (10.sup.10 ohms/square or more) to achieve high resolution, and the impedance must be low (less than that of the liquid crystal layer) so that substantially all of the voltage falls across the liquid crystal instead of the light blocking layer and to achieve a large voltage swing across the liquid crystal for a good dynamic range. In fact, the lower the impedance of the light blocking layer, the better the dynamic range that can be achieved.
In the prior art, Aoki's sheet resistivity is not a concern because the light blocking material is disposed in individual separate and distinct elements below each pixel. Charge spreading does not occur when the elements are separated, and therefore maintaining resolution by sheet resistivity and preventing charge spreading is again not addressed. In the present invention the light blocking layer is a continuous sheet across the light valve in which sheet resistivity must be kept high to maintain resolution.
In the prior art of Aoki et al, impedance is not a concern. The impedance of the light blocking layer does not need to be small because the electric field created by the pixel electrodes will not be crossing the liquid crystal material in the region where the light blocking layer is located, and therefore no reduction in dynamic range would occur. Because the impedance and sheet resistivity are not a concern, the necessary optical density of light blocking layer can be achieved by depositing an arbitrarily thick layer. In summary, there is no requirement for Aoki et al to have a thin light blocking layer.
Furthermore, the optical density required by Aoki et al is not discussed. The optical density required by Aoki et al may be less than that required by the present invention. In Aoki, the optical density is sufficient to reduce the ambient read light so as to allow proper functioning of the circuit. In the present invention, the optical density must be sufficiently high to make sure that the intense projection read light is reduced so much that the dim write light is not washed out by the read light. Thus, the optical densities required by these two applications are not necessarily the same.
A further matter not discussed in the prior art is the ratio of optical density per unit thickness. Part of the present invention is the ability to create the necessary optical density with a reduced thickness. As argued above, there is no necessity in Aoki et al to achieve a high optical density, while at the same time reducing thickness. As mentioned above, in the prior art of Sterling the thickness of the light blocking layer is described as 2 microns (column 3, line 59). Assuming that the optical density of Sterling is about the same as the present invention, because they have similar operating requirements, then Sterling has a ratio of optical density to thickness of (3-5 OD/2 microns) 1.5-2.5 OD per micron. In the present invention, the thickness of the light blocking layer is approximately 1 micron or less. Thus the ratio of optical density to thickness in the present invention is (3-5 OD/1 microns) approximately 3 OD per micron or greater.
The present invention is a light blocking layer two or more times thinner than the light valve of the prior art which achieves similar light valve gain and which is easier to deposit. Consequently, for a given level of light valve gain and given liquid crystal cell structure, the present light valve has a larger dynamic range and better resolution relative to the light valves of the prior art.